I used to get 3-4 Streetpasses per week back when the 3DS was still new and popular. Sometimes more. I don't think I have Streetpassed anyone in like 3 years, even if I still regularly play when on the bus.
I sometimes take my 3ds with me to London when I go, and I tend to get a few then. Especially when I went to the Pokemon Centre there loads of people had the same idea! I mainly used to get my Streetpasses from one of the hubs they had (which saved the data of the last 10 visitors) at a fast food place, but that doesn't exist anymore so much less now.
Years back I used to get at a few Streetpasses almost every day I took it with me (which was... most days). I also went to several cons a year (still do) and got hundreds of Streetpasses almost every day - really helped with the Streetpass games!
Today... I'm not sure since I haven't played it a lot for a few years up until recently, but I suspect I wouldn't really get any. Possibly at cons...
I used to have a literal army of streetpassers I've passed by. I think about 100+ or even higher I'm not sure. Now the only one I have is my brother...I miss getting them. :c I should take my 3ds more places with me again.
When I was in high school I would bring my ds and leave it in it's case and I got street passes all the time. Now, I don't get them that often but I also don't bring it with me to as many places anymore.
omg before my old nintendo broke, i had over 400 from more than 15 countries. i was so proud and then everything got deleted. now i have like 20 and only because of my mom. i'll never get the streetpass badge!